In the My Profile section, users have the ability to:
- Change the currency symbol
- Turn on Sales Tax
- Turn on auto-calculation of sales tax
- Set a default tax rate
To change or set any of these items, navigate to My Profile within the account settings menu in the left-hand navigation menu (look for your email address!):
Here you can change any of the above options using the edit 
button:
These will now be applied to invoices and quotes.